Drop foot jacks (also known as vehicle landing gear or landing gear jacks) are commonly used to lift and suspend vehicles, trailers, and other large bodies into elevated positions, with examples of such jacks being illustrated in U.S. Pat. No. 5,904,342 to Laarman and U.S. Pat. No. 5,423,518 to Baxter et al. Brief descriptions of these jacks will now be described so that the reader may better understand the structure and function of drop foot jacks.
U.S. Pat. No. 5,904,342 to Laarman illustrates a drop foot jack wherein an elongated drop foot is telescopically borne within an elongated sleeve, and a crank mechanism is interposed between the drop foot and sleeve so that winding a crank will cause the drop foot to extend from or retract within the sleeve. As a result, when the sleeve engages a trailer (or other object to be lifted) with the drop foot engaging the ground, rotating the crank allows the drop foot to raise or lower the trailer.
U.S. Pat. No. 5,423,518 to Baxter et al. illustrates a drop foot jack similar to that of Laarman, but wherein a potentially greater range of extension is allowed. As in Laarman, an elongated drop foot is telescopically borne within an elongated sleeve, but this sleeve is provided as an inner sleeve which is itself telescopically situated within an outer sleeve. A crank-driven screw engages the inner and outer sleeves so that they may be extended and retracted with respect to each other. Further, the drop foot may be extended or retracted within the inner sleeve, and apertures on the inner sleeve and drop foot may be aligned to allow pins to be inserted to engage the inner sleeve and drop foot together. Thus, a user may extend the drop foot with respect to the inner sleeve to some desired length and then engage them together, and may then crank the inner sleeve with respect to the outer sleeve to attain further height adjustment.
Several problems are perceived as being common in prior drop foot jacks. One significant problem is that in multi-stage drop foot jacks (i.e., those having multiple extending sections, as in Baxter), the mechanisms provided for locking the drop foot to the inner sleeve can provide less than optimal results. These mechanisms must often be compact owing to the telescoping arrangement of the drop foot jack and the need for easy engagement and disengagement, but a compact design tends to cause weakness insofar as pins (or other structure) bridging the inner sleeve and drop foot are subject to shear failure. As a result, the locking mechanism tends to impose the primary limitation on the jack's load capacity. Another significant problem is that drop foot jacks generally operate in harsh environments--they are exposed to road conditions, and therefore experience wide temperature variations and exposure to road grime, snow, salt, etc.--and they can therefore experience early failure. In large part, this arises owing to difficulties in keeping the moving parts of the jacks properly lubricated.